fix: preserve SQL bulk edge semantics

[j6k4m8]

Summary

• create missing endpoint nodes during SQL bulk edge insertion
• accept both two-item and metadata-bearing edge tuples
• merge metadata when a bulk edge already exists
• apply shared attributes consistently
• keep endpoint creation, inserts, and updates in one atomic transaction
• roll back the entire batch when insertion fails

[codspeed-hq]

Merging this PR will not alter performance

✅ 8 untouched benchmarks

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_{Comparing fix/sql-bulk-edges-71 (3b09d7e) with master (c102c1b)}